1. Field of the Invention
The invention relates primarily to the field of solid state high power circuit breakers.
2. Description of Related Art
A solid state circuit breaker (SSCB), especially for high voltage 3 phase applications, has been a dream of power engineers for many years. The existing mechanical breakers used today have to live through some major transient conditions. Some are:
1. Lightning strikes/voltage transients that generate very high voltages.
2. Extremely high short circuit current before the breaker has time to open.
3. Very high inrush current when the breaker is first turned on.
4. High energy absorption during the time the mechanical breaker is opening and clearing the arcs. If the breaker opening takes place during a high short current interval, the energy is very high.
Because of these conditions and others the mechanical breakers are limited in the number of times they can operate before they fail. Further, there is significant maintenance cost.
The object of the present patent is the design of an SSCB that will function under harsh conditions and operate a very high number of times. A further object of this patent is to incorporate a soft start feature that limits the inrush current when the breaker is first turned on. A major area of application of this patent application is 3 phase high voltage applications. It also works with DC inputs (high and low voltages). Under some conditions this type of breaker is the only way of interrupting a stiff DC source quickly.
A typical rating for an SSCB would be to open in 1 to 30 microseconds (us) with 600 amps at 26,000 volts. The basic concepts covered in this patent will work at any voltage and current. Many problems have to be solved in order to design a SSCB.
The first problem that has to be solved when working with an SSCB is the delay time of the semiconductors being used. There is at least a three orders of magnitude difference in response time between FET's and GTO's. A FET's delay time can be as short as 10 nanoseconds where a GTO's delay time can be as long as 30 microseconds. (delay time being the time between when the semiconductor circuit starts to turn off and is supporting full voltage). The concepts of this patent work for tubes also (both gas and vacuum).
An SCR's delay time can be anything from 0.5 microseconds to several hundred microseconds depending on the SCR used and how it is commutated. The combination of an SCR and so called DC side type commutation circuit duplicates the action of a fast semiconductor switch when being turned off. (i.e. in the order of 0.5 microseconds the load is receiving no energy from the input power source--said otherwise--the SCR in conjunction with the DC side commutation circuit supports full voltage in 0.5 microseconds). The basic idea of DC side commutation is that the commutation voltage is induced in series with the SCR instead of in parallel.
The basic problem that a long delay time causes is that the current through the semiconductor will get too high before the switch can turn off. For example, if a 20,000 volt source (AC or DC) was short circuited the di/dt could easily be 10,000 amps per microsecond. If just solid state switches rated for 600 amps and a delay time of 1 microsecond were used in series with the voltage source then the current would be 10,000 amps before the switch was off. Most practical 600 amp switches can not handle a 10,000 amp surge during the turn off process. The problem becomes how do you slow down the di/dt without affecting the normal operation of the SSCB and the circuit it is protecting. The higher the input voltage the more important the delay time problem becomes because the higher the di/dt can be with a dead short.
Another problem that has to be solved is how do you protect the semiconductors when lightning or other high voltage transients are injected into the SSCB system. Lightning strikes can cause 150,000 volt spikes with a rise time of 0.4 microseconds and currents of 100,000 amps for 50 microseconds time duration. It is not practical to rate most high voltage SSCB's to handle voltages and currents of these magnitudes.
A third problem is how do you absorb the energy stored in the inductance of a power system. Said otherwise, if you turn off the SSCB very fast, how do you absorb the energy stored in the system inductances without causing high voltages on the semiconductors themselves?
Another major problem that has to be solved is how do you limit the inrush currents in the SSCB when the SSCB is first turned on and still get the SSCB fully on in a short time (i.e. an effective soft start method). Normal inrush currents are 10 to 30 times the rated current of a breaker. Without a soft start feature the cost of the SSCB would be very high or some sort of compromise would have to be made in the operation of the SSCB.